A manual co-ordinate measurement (CMM) arm, well known in the art, is an instrument formed from a plurality of tandemly connected segments whereby the last segment has a large range of motion and its position and orientation can be determined with accuracy through encoders which measure the angles adopted between the segments. They are manufactured by Faro Inc., Cimcore or Metris, and employed in the measurement of objects. A CMM arm is made of several segments, usually 6 or 7, each linked by joints allowing rotation of the segment around one or more axes.
For some joints, it is important that the degree of rotation is without limit because an operator may wish to take advantage from rotating the segment in one direction when performing a measurement without encountering any mechanical end stop. For example, when a laser scanner is attached to the last segment of the CMM arm, it is desirable to have the last two segments in a coaxial arrangement, and the joint between these two segments disposed with an infinite degree of rotation i.e. the joint can make one or more complete turns from 360 deg. U.S. Pat. No. 5,829,148 describes a manual CMM arm where the rotation of some joints is unlimited.
On the other hand, there are certain joints for which unlimited rotation is not essential, because the operator does not need to infinitely rotate the axis in one direction when performing a measurement. Therefore, he would not encounter any mechanical end stop at either end of a stroke during normal operations.
When an infinite rotation connection device is needed, well-known slip ring devices which are electrically conductive, are used instead of cables to transmit the electrical signals and power between segments. The slip rings relying on mechanical contact with a brush, however, are subject to wear and tear. Further, they are prone to interference which reduces the quality of the signal and may reduce the accuracy of the arms. Slips rings are also expensive and fragile.
The use of slip rings is avoided for joints having a limited number of rotations. In this this case, a better accuracy is obtained. The accuracy of joints with limited rotation is further improved because the mechanical contact between successive segments can be achieved using ball bearings (or any other type of bearing). In addition, cabling is sufficient to transmit power and/or data thereby reducing the problem of interference. Moreover, the cost of the joint is reduced.
For joints with a limited number of rotations, it is usually sufficient to have approximately one full rotation, so that the operator can reach any angle in the range 0 through 360 deg; nevertheless, in certain cases it is better to reach an angle higher than 360 deg. A total rotation angle higher than 360° can be achieved e.g. with multiple rotation rings. U.S. Pat. No. 6,931,745 describes such a device.
When the rotation is at its limit, one end stop attached to the first segment comes in contact with a second end stop attached to the second segment. The contact between the end stops produces a collision and a shock that are unpleasant for an operator. Moreover, the shock introduces stresses on the segments or the joint which can reduce the accuracy of the device. Moreover, a shock carrying excessive force may physically damage the joint.